1. Field of the Disclosure
The present disclosure relates to a liquid crystal display device (LCD), and more particularly, to an LCD which uses a light emitting diode (LED) and has excellent stability and color reproduction range for temperature change.
2. Discussion of the Prior Art
A liquid crystal display devices (LCDs), which has advantage in displaying moving images and high contrast ratio and is widely used for TV, monitor, or the like, displays images using optical anisotropy and polarization of liquid crystal.
The LCD uses a liquid crystal panel having two substrates facing each other and a liquid crystal layer therebetween as a main component, and alignment direction of liquid crystal molecules is changed by an electric field in the liquid crystal panel to realize difference of transmittance.
However, since the liquid crystal panel does not have a self-luminescent element and requires a light source, and to do this, a backlight unit having a light source is located below the liquid crystal panel.
The backlight unit is categorized into a direct type and an edge type according to arrangement structure of lamp.
The edge type backlight unit has a structure that at least one light source is located at one or both sides of a light guide plate. The direct type backlight unit has a structure that light sources are located below the liquid crystal panel.
The direct type backlight unit has a limit of thin profile and is thus mostly used for an LCD considering brightness as being more important than thickness. The edge type backlight unit is able to have a thin profile and is thus mostly used for an LCD, for a monitor of a laptop or desktop computer, considering thickness as being important.
Recently, a thin-profile LCD has been actively researched, and thus the edge type backlight unit has also been actively researched.
Particularly, an LED package is widely used as a light source of the edge type backlight unit because the LED package has advantages of a small size, a low power consumption and a high reliability.
FIG. 1 is a cross-sectional view illustrating an LCD including an edge type backlight unit using an LED package according to the prior art, FIG. 2 is a view illustrating an LED PCB having the LED package mounted thereon, FIG. 3 is a cross-sectional view illustrating an LED package according to the prior art, and FIG. 4 is a schematic block diagram of a driving circuit of the LED driving circuit board according to the prior art.
With reference to FIGS. 1 and 2, the LCD 1 includes a liquid crystal panel 10, a backlight unit 20, a main support 30, a bottom cover 50, and a top cover 40.
The liquid crystal panel 10 includes first and second substrates 12 and 14 attached to each other with a liquid crystal layer therebetween. Polarizing plates 19a and 19b are located on outer surfaces of the first and second substrates 12 and 14, respectively.
The backlight unit 20 is below the liquid crystal panel 10.
The backlight unit 20 includes an LED assembly 70 including an LED PCB (printed circuit board) 75 and LED packages 72 mounted on the LED PCB 75, a reflection plate 25 on the bottom cover 50, a light guide plate 23 on the bottom cover 50, and a plurality of optical sheets 21 on the light guide plate 23.
The liquid crystal panel 10 and the backlight unit 20 are surrounded by the main support 30 of a rectangular frame shape and coupled with the top cover 40 and the bottom cover 50, and thus the LCD 1 is manufactured.
A LED driving circuit board 79 to drive the LED package 72 is located below the bottom cover 50, and is electrically connected to the LED PCB 75 through a wiring. To protect the LED driving circuit board 79, a protection cover 90 is located corresponding to the LED driving circuit board 79.
In the LED assembly 70, the LED packages 72 are mounted on the LED PCB 75 being spaced apart from each other and function as light sources.
With reference to FIG. 3, the LED package 72 includes three LED chips 74a, 74b and 74c emitting red (R), green (G) and blue (B), respectively, a lead frame 76 having the LED chips 74a, 74b and 74c mounted thereon, a package housing 77 that has a partition wall on the lead frame 76 and reflects lights emitted from the LED chips 74a, 74b and 74c, and a transparent organic layer 78 filling a space surrounded by the package housing 77.
Accordingly, the LED package 72 including the three LED chips 74a, 74b and 74c finally emits a white light W through the mixture of red, green and blue lights. The white light W enters the light guide plate 23 through a light incidence surface of the light guide plate 23, then is refracted toward the liquid crystal panel 10, then is processed along with a light reflected by the reflection plate 25 into a high quality plane light while passing through the optical sheets 21, and then is supplied to the liquid crystal panel 10.
The LED driving circuit board 78 includes a driving circuit to drive the LED package 72.
With reference to FIG. 4, the driving circuit of the LED driving circuit board 78 includes a DC-DC block E1 to supply DC powers independently to the red, green and blue LED chips 74a, 74b and 74c, a PWM (pulse width modulation) control IC (integrated circuit) E2 to control the output currents of the DC-DC block E1, and a color controller E3.
The driving circuit is connected to the red, green and blue LED chips 74a, 74b and 74c through the LED PCB 76, and is also connected to a color sensor 84.
The color controller E3 senses a color coordinate of the white light emitted from the LED package 72, and calculates an error with respect to a target color coordinate and an output compensation amount, and transfers the output compensation amount to the PWM control IC E2.
Employing the color sensor 84 and the color controller E3 in the prior art LCD 1 is for preventing change of a color temperature of a white light that is caused as times go on because the red, green and blue LED chips 74a, 74b and 74c have different temperature properties.
Further, each LED package 72 has the red, green and blue LED chips 74a, 74b and 74c therein, and the LED chips are independently driven. With reference to FIG. 5 that is a graph illustrating light intensities of the red, green and blue LED chips according to temperature rise, it is shown that the red, green and blue LED chips 74a, 74b and 74c have different light intensities according to temperature rise, and this means the red, green and blue LED chips 74a, 74b and 74c have different dependences on temperature.
Accordingly, considering the different dependences on temperature among the LED chips 74a, 74b and 74c, in order that a constant white light without change on a color coordinate is to be emitted even though temperature is changed, the color controller E3 is required to have a complicated circuit configuration.
In other words, the color sensor 84 measures in real time a color change of the white light emitted from each LED package 72, and more precisely, changes of red, green and blue emitted from the respective LED chips 74a, 74b and 74c, and the color controller E3 compares a target color coordinate of a white light that is a mixture form of red, green and blue with a color information value measured by the color sensor 84, and calculates a difference between the target value and the measured value.
The color controller E3 calculates an output compensation value of the PWM control IC E2 to offset the difference between the target value and the measured value, and transfers the compensation value to the PWM control IC, then the PWM control IC E2 transfers the output compensation value to the DC-DC block E1, and then the DC-DC block E1 applies current values corresponding to the difference between the target value and the measured value to the red, green and blue LED chips 74a, 74b and 74c, respectively. Accordingly, even though change of temperature happens, a white light of a constant color coordinate can be achieved.
Accordingly, since the color control for change of color coordinate is required to be always conducted for each of red, green and blue, configuration of the LED driving circuit of the LED driving circuit board 79 is very complicated. Further, since the three LED chips 74a, 74b and 74c emitting red, green and blue lights, respectively, are all mounted in each LED package 72, and the color sensor 84 measuring the intensity change of the red, green and blue lights is necessarily required, a production cost of the LCD 1 increases.